Topologies
| Type | Transfer time | Output on mains | Use |
|---|---|---|---|
| Offline / standby | 4–25 ms | Filtered mains | Home PC, light loads |
| Line-interactive | 2–10 ms | AVR-regulated mains | Small business |
| Double-conversion (online) | 0 ms | Synthesized from inverter always | Data center, sensitive gear |
Sizing
| VA vs W | VA = V · I (apparent); W = real. Size UPS so both ratings handle the load |
|---|---|
| Power factor | Old PSU PF ≈ 0.6; modern PFC ≈ 0.95. Match UPS rating accordingly |
| Run-time | Battery Wh / load W. Derate ~15% for inverter efficiency |
| Headroom | Size UPS 1.2–1.5× expected peak for surge |
Battery
- Sealed lead-acid (SLA/VRLA): cheap, heavy, ~3–5 year life. Most consumer UPSes.
- LiFePO₄: 10+ year life, lighter, higher cost. Common in newer UPSes.
- Replace battery when runtime drops below ~80% of original.
- UPS batteries degrade faster in hot environments — rule: 10 °C rise halves life.
Rules of thumb
| PC + monitor | 200–400 W typical |
|---|---|
| Server | 250–800 W per 1U |
| Home network gear | 30–80 W |
| Home NAS (4-bay) | 40–80 W |
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